1. Field of the Invention
The present invention relates to a spread spectrum receiver, and more specifically, to a spread spectrum receiver having the function of controlling a symbol detection window, and a method thereof.
2. Description of the Background Art
Conventionally, one type of the spread spectrum communication receivers employs a digital matched filter as a measure of demodulating a received spread spectrum signal into symbol data. The digital matched filter functions as taking a correlation of an input signal with a sequence of transmission spread codes to output a correlation value. In order to restore symbol data from the correlation value, a correlation threshold is set, so that when the correlation value exceeds the correlation threshold it is estimated that a spread code sequence is transmitted which is associated with one of the correlators which outputs a correlation value indicating the maximum of the correlation peaks, that spread code sequence being used to demodulate the received symbol data. A variety of technologies have been disclosed concerning the detection window of the spread spectrum communication. Japanese patent laid-open publication No. 2002-57602, for example, discloses a method of detecting a slot timing from spread code sequences. Another Japanese patent laid-open publication No. 2004-153692 discloses a synchronous position detection method for detecting a synchronous position with the detection window made as small as possible for an accurate synchronous position determination.
The conventional methods have problems, however, that in a high noise environment, any input signal such as noise other than an intended input signal may cause a correlation value to exceed a threshold to provide the maximum correlation peak value, thus causing false data to be determined as appropriate received symbol data.
More particularly, the symbol detection window is open with a predetermined sectional width, such as a width corresponding to several chips of a spread code sequence. During a period when the symbol detection window is open, a poor communication status with a high noise level may allow correlation values corresponding to a plurality of spread code sequences to exceed the threshold and exhibit the correlation peak values. This case raises a problem where it is erroneously determined that the spread code sequence of an intended symbol correlation peak value is transmitted, thus providing a symbol detection error.
Under a poor communication condition with a high noise level, an intended signal input to a digital matched filter may not cause the correlation value to exceed the threshold, thus failing to determine a symbol. In that case, the threshold may be set lower so as to allow for the symbol detection at the expense of possible high detection error.